Since substances absorbed by the intestine must traverse the apical cell membrane, the functional activity of this critically located structure must determine the nature of all absorptive processes. In this project microvillous membranes will be isolated from absorptive cells by methods previously shown by us to yield satisfactory preparations. Functional activity of these isolated membranes will be examined in relation to nutrients known to be transported across the intestine by specific transport mechanisms. Initial efforts have been directed at examination of the attachment of intrinsic factor-vitamin B12 complex to specific membrane receptor sites. Investigations are designed to elucidate the precise location and nature of these receptor sites, the mechanism of substrate attachment, and the relation of membrane binding to subsequent transport across the intestine. In addition, the binding of glycoproteins other than IF to microvillous membranes will be examined. Studies will be undertaken to solubilize, isolate and characterize the microvillous membrane receptor for IFB12 complex and to examine the membrane proteins present in intestinal microvilli. As part of this work we will also examine glycoprotein synthesis and secretion by intestinal mucosa in vitro and IF synthesis and secretion by gastric mucosa in vitro. Studies of vitamin B12 absorption will be emphasized and will include: investigations into the enterohepatic circulation of vitamin B12, the role of the pancreas in B12 absorption, entry of B12 into the ileal cell, the nature of the active process involved in transcellular transport and the relation of transcobalamin II to the absorption of the vitamin.